The invention described herein may be manufactured, used, and licensed by or for the United States Government for governmental purposes without the payment to me of any royalty thereon.
This invention relates to the generation and radiation of microwave energy.
The generation of microwaves using IR photoconductive (PC) switching has been around since the 1970s. With the improvements in laser sources and semiconductor materials in the 1980s and 1990s, much research has been conducted into the generation of high-power microwaves (HPM) using the PC switch approach. The output power of the PC switch is a function of the bias voltage, the on-state resistance, and the load impedance. The upper frequency limit of the microwaves is a function of the risetime of the laser pulse. A 500-picosecond risetime laser, generates a microwave frequency spectrum that is ultra-wideband with about 2 GHz as the upper frequency limit. The bandwidth of the microwave spectrum is a function of the laser pulsewidth and the bandwidth of the antenna. It is usually limited by the bandwidth of the antenna. When gallium arsenide (GaAs) and silicon (Si) are the materials of the PC switch, the pulsewidth is usually less than 100 xcex7s with repetition rates no more than several hundred Hertz. The use of silicon carbide (SiC) or gallium nitride (GaN) will allow xcexcs pulsewidths and kHz repetition rates.
The Luneburg lens is a dielectric sphere or hemisphere, where the index of refraction varies with distance from the center of the sphere such that a point source incident on one face of the sphere is diverged to a parallel ray on the opposite face. Luneburg lenses have existed and been used in special purpose applications for over 50 years. They have been used primarily for radar reflector and antennas. Luneburg lenses have several important characteristics that can be exploited to produce a versatile microwave antenna. The efficiency of the Luneburg lens is above 75%, and can be as high as 90% at low microwave frequencies. The Luneburg lens antenna has excellent wide-angle scanning performance, good gain, and wide bandwidth over its range of performance. Its bandwidth is usually limited by the feed structure. Wide-angle scanning is realized by moving the feed point about the lens either mechanically, electrically, or a combination of the both. Since the lens has spherical symmetry, it can be scanned over 4xcfx80 steradians. Rays emerging from the feed point, do not illuminate uniformly across the aperture, but spread out from the center of the sphere in elliptical ray paths, and move out to give parallel rays emerging from the opposite surface of the sphere. The feed pattern is multiplied by a factor of sec(xcex1) to obtain the aperture illumination pattern, where xcex1 is the feed angle.
Luneburg lens can be fabricated by stacking dielectric sheets with hole and slot distributions such that at any given location within the lens, the local relative permittivity equals the square of the index of refraction prescribed by the classical Luneburg lens formula.
xcex5eff=2xe2x88x92(h2+r2)xc2xd/R,
where h is the lateral distance of the specific layer from the center of the sphere, r is the radial distance from the center of a specific layer, and R is the radius of the sphere. Another method for fabricating a Luneburg lens, which is often used for satellite antennas, is to fabricate concentric shells where the selection of dielectric constants and thickness of the concentric shells is a step-wise approximation to the classical Luneburg lens equation. The lenses are typically manufactured from either Polystyrene or from Polyethylene beads. The materials are lightweight in their expanded form, but when molded or compressed to obtain the desired density and hence dielectric properties they can become heavy. One can reduce the weight of the lens by introducing metal Fat particles, slivers, cubes, or ceramics. This inventor prefers the use of ferroelectric particles, since they can have excellent dielectric strengths, low-loss tangents, and high dielectric constants (xcex5r greater than 500). These characteristics for the embedded and host materials are compatible for handling large peak and average powers. The maximum frequency-of-operation places a limit on the shell thickness of about one wavelength or less to produce adequate gain and minimum manufacturing costs. Tradeoffs are made in the shell thickness, number of shells, materials used, and etc. to obtain the best antenna performance at minimal cost. For example, too many shells are difficult to construct, add cost, and can introduce air gaps between the shells. Air gaps can reduce the overall efficiency of the lens and defocus the beam, especially at high microwave frequencies. Present day lens designers using new sophisticated spherical wave modeling techniques can design profiles other then the classical Luneburg profile that varies xcex5eff from 2 at the center of the sphere to 1 at the outer surface. A design profile that varies xcex5eff from about 5 at the center of the sphere to 1 at the outer surface appears to be practical for producing a more compact, lighter antenna. These designs can be simulated and tailored to meet the antenna specifications prior to building the antenna.
The invention described herein is aimed at fulfilling the urgent military need for compact, high-gain/high-power sources and radiators that are rugged for the battlefield environments and are compatible for mobile, tactical platforms with DEWs and radars.
Briefly, the foregoing and other objects are achieved by using a semiconductor switch or an array of switches such as silicon carbide (SiC), gallium nitride (GaN), silicon (Si), or gallium arsenide (GaAs). The switch(s) are illuminated by laser energy that is in the infrared (IR) or ultraviolet (UV) spectra. Unlike microwave energy that is generated by a microwave tube, coaxial cable or waveguide is not required to transport the microwave energy to the antenna. In this invention, the photoconductive switch(s) is integral with the antenna, and is the feed structure for the antenna. Fiber optic cable is utilized to transport the IR or UV energy to the PC switch(s). In FIGS. 3 and 5, a PC switch is part of a bowtie antenna feed structure. High-voltage cables are used to bias the semiconductor switch. The switch in the off state behaves like an insulator. In the on state, the laser energy causes an impulse current to flow in the switch. The signature of the microwave radiation follows the fingerprint of the laser pulse. A fast risetime ( less than xcex7s) laser pulse will generate radiation in the microwave spectrum.
The bowtie feed structure is located on the outermost shell (the invisible xcex5r=1 shell of the classical Luneburg lens or other predetermined dielectric lens profile). A hemispherical Luneburg lens (FIG. 3) or an almost spherical Luneburg lens (FIG. 5) is mounted flush on a ground plane. The bowtie feed structure is mounted on a motorized stand such that the feed structure can be made to rapidly rotate 360xc2x0 in the azimuth direction and at least 90xc2x0 in elevation. The rapid wide-angle scanning feature of the inventive item is made possible because the microwave generator is the feed structure of the antenna, and waveguide or radio frequency (RF) cable is not required to transport the microwave energy. The IR or UV energy is transported from the laser source to the PC switch(s) by fiber optic cable. The fiber optic cable does not need to have a physical connection to the laser source. A small air gap between the laser source and fiber optic cable is utilized, and this air gap (if kept short) will not adversely decrease the laser energy to the switch(s). The dc cables that are required to bias the PC switch(s) need not be connected to the PC switch(s) until the feed structure is properly aligned with the Luneburg lens for irradiating the target. Then a contact switch is engaged to complete the electrical circuit and the switch is biased with the predetermined voltage. The switch is in the off state until the IR or UV energy illuminates the switch. Upon switch illumination, the microwave energy is generated and radiated a short distance from the outermost invisible xcex5r=1 shell of the Luneburg lens through each successive shell of the lens. For the spherical or almost spherical Luneburg lens embodiment, the rays emerge as parallel rays at the diagonally opposite point on the lens. For the hemispherical embodiment, the rays are reflected off of the surface of the ground plane, and the rays will follow the paths in accordance with Snell""s law. A virtual source is present on the other side of the ground plane, and therefore two beams are produced (one from the real source and one from the virtual source). The result is that the antenna""s effective aperture is double for the hemispherical configuration versus the spherical or almost spherical configuration. The hemispherical embodiment has the advantages of higher (about double) gain and smaller profile compared to the spherical or almost spherical embodiment. However, pointing and tracking is more complex, since the rays are reflected off of the ground plane.
This microwave generator/radiator using photoconductive switching and dielectric lens has benefits over previous art. PC switch antennas of previous art shown in U.S. Pat. Nos. 5,596,438, 5,491,490, 5,351,063, 5,319,218, 5,513,056, 5,283,584, 5,280,168, 5,262,657, and 5,227,621 do not have the capability of pointing, tracking, and scanning over 360xc2x0 in the azimuth direction and 180xc2x0 in elevation. A phase array scheme would be required to obtain wide-angle scanning, but the complete 360xc2x0 in the azimuth direction and 180xc2x0 in elevation coverage would still most likely not be possible and if it were possible, it would be a complicated, high-cost technique. Luneburg lens of previous art utilize microwave sources that transport the microwave energy from a source to the antenna feed structure via coaxial cable or waveguide. This requires the microwave source to rotate with the feed structure. For HPM applications, this limits the scanning speed, and 4xcfx80 steradians coverage of the antenna. This invention overcomes these limitations because the microwave generator is the feed structure of Luneburg lens antenna.